My outdoor is about the same length. I do a lot of one-stride gymnastics and singles since my horse is green.
Lines don't need to be drilled. If they can keep themselves together in a tiny ring, riding in a large ring is a cakewalk.
Before I go out and about to a place with lines, I do some poles on the ground in 12' increments so I remind myself what canter the horse needs to get the strides. Then subtract a little bit because a bigger space and jumps often open the step up a little more.
I ride in a very small indoor at school and we set up at lot of singles and do long/short approaches our courses are usually 3 jumps we do in all directions. It actually helps when I show because then the bigger rings don't require as much work!
Lots to do. Gymnastics of one stride and two. Oxer, oxer on the outsides. Be blessed with a tiny indoor. It will make you a great rider in the corners. Anyone can ride in big riders, but it will prepare you for fences that come up early
If you set up lines and grids on the diagonal you have a little extra space to play with. The indoor at our farm is about the same size as yours, OP, and it's very doable. Our kiddies do very well at jumper shows because they're used to tight turns....although they don't always know what to do with the extra space!
Grids are great in the small ring and a diagonal jump is a great thing to school. The indoor I ride in at home is 60' X 160' and we can have about 5 jumps or so set at once. Be glad you don't have the opposite problem, horses and riders used to a huge space often struggle with the smaller areas and short approaches. It's also great to practice halting in the corner in a small ring because it forces you to get it done. The only thing I've found to be difficult in the small ring is a lead change so for the most part we do the simple change at home, only schooling the flying when we've set a jump with a long landing side.